This disclosure relates to a machine, such as a linear actuator or a rotary machine, having one or more cylinders each having an inner surface comprising a one or more recesses indented into the inner surface, and in particular relates to an inner surface of a cylinder having a plurality of recesses provided only in predetermined regions of the cylinder bore.
The term “rotary machine” is intended to encompass reciprocating machines such as internal combustion engines, compressors and vacuum pumps, as well as machines with rotating components but no reciprocating parts.
An internal combustion engine typically has one or more reciprocating pistons which are lubricated to reduce the friction as the piston slides within a cylinder bore. Lubricated sliding contacts, such as between the piston rings of a piston and an inner surface of the cylinder bore, have frictional losses due to the shear forces generated in the lubricant, contact between surface asperities, and boundary contacts caused by additives in the lubricant.
In some situations there is an operational clearance between a circumferential surface of the piston and the inner surface of the cylinder bore, and it is desirable to maintain contact only between the inner surface of the cylinder bore and the piston rings. As a result of such a configuration, the body of the piston can move, or rock, from side to side during operation, for example when the piston moves between an upward and a downward stroke, and vice versa.
Such movement can cause the piston to damage the inner surface of the cylinder bore, for example at the point where the piston “slaps” over at top dead center of the piston stroke. It is desirable, therefore, to mitigate damage to the inner surface of the cylinder bore.
According to an aspect of the present disclosure there is provided a machine, for example an engine, the machine being such that a grade point of the body portion of a piston of the machine periodically engages an inner surface of the cylinder during operation of the machine, this engagement defining a grade point contact zone between the body portion of the piston and the inner surface of the cylinder when the piston body rotates about an axis of a piston pin, the inner surface of the cylinder having a bearing region comprising one or more recesses indented into the inner surface, the bearing region being smaller than the inner surface of the cylinder and being substantially centered on the grade point contact zone.
In the context of the present disclosure, the term “engage” is intended to encompass two surfaces which are separated by a thin film of lubricant, as well as surfaces which come into direct physical contact.
The form, e.g. shape and/or size, of the bearing region may be defined, at least in part, by a first dimension in the direction of travel of the piston. The first dimension may be a function of a dimension of the body portion in the direction of travel of the piston. For example, the first dimension may be a function of a dimension of a piston skirt of the piston in the direction of travel of the piston. The ratio of the first dimension to the dimension of the body portion in the direction of travel of the piston may be in a range of approximately 1:2 to 1:1. For example, the piston skirt may be approximately 40 mm in length, i.e. in the direction of travel of the piston, and the first dimension, i.e. the length of the bearing region, may be in the range of approximately 20 mm to 40 mm. The first dimension may be approximately 30 mm.
The form of the bearing region may be defined, at least in part, by a second dimension in the circumferential direction of the piston. The second dimension may be a function of a dimension of the body portion in the circumferential direction of the piston. For example, the second dimension may be a function of a dimension of a piston skirt of the piston in the circumferential direction of the piston. The ratio of the second dimension to the dimension of the body portion in the circumferential direction may be in a range of approximately 1:4 to 1:2. For example, the piston skirt may have a circumferential dimension, i.e. an arc length, of approximately 60 mm, and the second dimension, i.e. circumferential dimension of the bearing region, may be in the range of approximately 15 mm to 30 mm. The second dimension may be approximately 22 mm.
The inner surface of the cylinder and at least a portion of a circumferential surface of the piston body may be parallel in the contact zone, for example during operation of the machine. The piston body and/or the inner surface may be configured to deform elastically under loading. The portion of the piston body that deforms elastically under loading and engages the inner surface of the cylinder may define an elastic contact zone between the inner surface of the cylinder and the circumferential surface of the piston body. The dimension of the contact zone in the direction of travel of the piston may be defined by a dimension, for example the axial length, of the elastically deformed portion of the piston body. The circumferential surface of the piston body and the inner surface of the cylinder may be parallel as a result of the elastic deformation of the piston body and/or the inner surface. The dimension of the recess in the direction of travel of the piston may be less than the dimension of the elastically deformed portion of the piston body in the direction of travel of the piston. The first dimension may be a function of the dimension of the elastically deformed portion of the body portion of the piston in the direction of travel of the piston. The first dimension may be a function of the dimension of the elastically deformed portion of the inner surface of the cylinder in the direction of travel of the piston. The second dimension may be a function of the dimension of the elastically deformed portion of the body portion of the piston in the circumferential direction of the piston. The second dimension may be a function of the dimension of the elastically deformed portion of the inner surface of the cylinder in the circumferential direction of the piston.
The configuration of the contact zone may depend on the operational state of the engine. For example, at a high engine output the forces acting on the piston body may be higher than the forces acting on the piston body at a lower engine output. As such the form of the contact zone, e.g. the elastic contact zone, may vary depending on the output of the engine. The form of the elastic contact zone may be dependent upon the axial and/or radial loading of the piston body against the inner surface, the form of the circumferential surface of the piston body, and/or the material properties, e.g. the Young's modulus, of the respective surfaces.
A lubricant may be used to reduce the friction between the piston body and the inner surface of the cylinder. A lubricant film may be formed in the contact zone between the circumferential surface of the piston body and the inner surface of the cylinder during operation of the engine. The lubricant film in between at least a portion of the circumferential surface and the inner surface may have a film thickness that is substantially constant in the direction of travel of the piston during operation of the machine. For example, the film thickness of the lubricant film may be substantially constant where the circumferential surface of the piston body and the inner surface of the cylinder are parallel. The portion of the lubricant film that has a substantially constant film thickness may have a dimension in the direction of travel of the piston, for example an axial dimension that defines the overall length of the portion of the lubricant film that has a substantially constant film thickness. The dimension of the recess in the direction of travel of the piston may be less than the dimension of the portion of the lubricant film that has a substantially constant film thickness in the direction of travel of the piston.
The inner surface may comprise a top bearing region having a plurality of recesses indented into the inner surface. The top bearing region may be provided on the thrust side of the cylinder. The top bearing region may extend towards the bottom end of the cylinder away from a contact zone between a bottom piston ring and the inner surface when the piston is at top dead center of a stroke.
The inner surface may comprise a bottom bearing region having a plurality of recesses indented into the inner surface. The bottom bearing region may be provided on the anti-thrust side of the cylinder. The bottom bearing region may extend towards the top end of the cylinder away from a contact zone between a top piston ring and the inner surface when the piston is at bottom dead center of the stroke of the piston.
The inner surface may be an inner surface of a bore of a cylinder block. The inner surface may be an inner surface of a cylinder liner.
The recesses may be configured to retain a fluid, for example each recess may comprise a pocket configured to trap the fluid in the inner surface. The recesses may be configured to slow down the rate at which fluid drains away from the top and/or bottom bearing regions of the inner surface. The top bearing region and the bottom bearing region may be separated by a middle region having no recesses indented into the inner surface. The top bearing region and the bottom bearing region may be spaced apart, for example by the middle region, in the direction of travel of the piston.
The top bearing region may comprise a plurality of recesses extending partially around the full circumference of the inner surface. The bottom bearing region may comprise a plurality of recesses extending partially around the full circumference of the inner surface. The top bearing region may have an axial dimension in the direction of travel of the piston. The bottom bearing region may have an axial dimension in the direction of travel of the piston. The middle region may have an axial dimension in the direction of travel of the piston. The axial dimension of the middle band may be greater than the axial dimension of the top and/or bottom bearing regions.
The bearing region, for example the top and/or bottom bearing regions, may be elliptical in shape. However, the bearing region may have any appropriate shape, such as a diamond, a square, a rectangle or a triangle, amongst others. The top and bottom bearing regions may be similar in form. The top and bottom bearing regions may be different in form.
The bearing region may have a center point located in a radial plane of the piston when the piston is at top dead center. The radial plane may be parallel to the axis of rotation of a piston pin of the piston. The center point may be located in a radial plane within the range of approximately 0 to 20 mm from a bottom edge of the body portion of the piston, when the piston is at top dead center. The center point may be located in a radial plane located at approximately 16 mm from the bottom edge of the body portion of the piston, when the piston is at top dead center. The radial plane may be located at a gauge point of the piston.
The bearing region may have a center point located in an axial plane of the piston when the piston is at top dead center. The axial plane may be perpendicular to a rotational axis of a piston pin of the piston and/or a rotational axis of a crankshaft of the machine. The axial plane may be a mid plane of the piston, for example the axial plane may be coincident with the mid point of the rotational axis of the piston about the piston pin.
The bearing region may have a center point located at or near to a point on the inner surface that corresponds to a grade point of the piston when the piston is at top dead center or bottom dead center. In the context of the present disclosure, the term “grade point” refers to the portion of the circumferential surface of the piston body that first engages the inner surface of the cylinder when the piston rotates about the axis of a piston pin, for example as the piston rocks at top dead center or bottom dead center. The grade point may correspond to the largest radial diameter of the piston body. The piston body may have a radial dimension in a direction perpendicular to the piston pin that is larger than a radial dimension in a direction parallel to the piston pin. The piston body may taper such that the diameter at a lower portion, e.g. a piston skirt, of the piston body is larger than the diameter of an upper portion, e.g. a piston head, of the piston body. In this manner, the operational clearance between the inner surface of the cylinder and the lower portion of the piston body may be less than the operational clearance between the inner surface of the cylinder and the upper portion of the piston body, for example when the engine is cold.
The shape, for example an axial and/or radial cross-sectional shape, of the piston may be selected to compensate for expansion of the piston and/or the physical forces applied to the piston during operation of the engine. During operation of the engine, the upper portion of the piston body may expand by a greater amount than the lower portion of the piston body as the engine temperature rises, since the upper portion of the piston body is closer the combustion of the fuel in the cylinder. The difference in the operational clearance between the inner surface of the cylinder and the lower portion of the piston body, and the operational clearance between the inner surface of the cylinder and the upper portion of the piston body may decrease as the temperature of the engine increases, for example as the upper portion of the piston expands. As a result, the circumferential surface of the piston body may become more parallel with the inner surface of the cylinder as the engine heats up. The risk of damage to the inner surface of the cylinder caused by piston slap may be higher during a warm-up phase of the engine when the engine is cooler and operational clearances are larger, which allow the piston to rock by a greater amount. It is advantageous, therefore, to provide the enhanced bearing region in one or more areas of the inner surface of the cylinder that may become scuffed as a result of the contact with the body portion of the piston.
The recesses may be arranged into rows that extend circumferentially around the inner surface. The recesses of each row may have a circumferential spacing between adjacent recesses. The ratio of the dimension of the recess in the circumferential direction to the dimension of the circumferential spacing may be approximately 2:1. One of the rows may be offset from another of the rows in the circumferential direction by an offset distance. The ratio of the dimension of the recess in the circumferential direction to the dimension of the offset distance may be approximately 2:1. The rows may be adjacent.
There is provided a vehicle comprising one or more of the above mentioned machines.
According to another aspect of the present disclosure there is provided a method of forming a machine having one or more cylinders, the configuration of the machine being such that a grade point of a body portion of a piston of the machine periodically engages an inner surface of the cylinder during operation of the machine, this engagement defining a grade point contact zone between the inner surface of the cylinder and the body portion of the piston when the piston body rotates about an axis of a piston pin, the inner surface having an enhanced bearing region comprising one or more recesses indented into the inner surface, the enhanced bearing region being smaller than the inner surface, the method comprising:
determining the location of the grade point contact zone; and
providing the enhanced bearing region in a region of the inner surface that is substantially centered on the grade point contact zone.
To avoid unnecessary duplication of effort and repetition of text in the specification, certain features are described in relation to only one or several aspects or arrangements of the disclosure. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or arrangement of the disclosure may also be used with any other aspect or arrangement of the disclosure.